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Sump & Seal

Crawl spaces: seal the ground, then deal with the water

A crawl space is a basement with worse access and the same physics. Most of the mistakes down there come from sealing it before removing the reason it is wet.

Bare soil in a crawl space is an evaporation surface. Even in dry weather it releases a surprising amount of water vapor, and because warm air rises through a house, a large share of that ends up in the rooms above — which is why a musty crawl space is a whole-house smell. Covering the ground with a sealed vapor barrier is the single highest-value thing you can do down there.

But sealing is not drainage. A barrier over standing water traps a pond under plastic, and an encapsulation kit will not tell you that on the box. If water collects in your crawl space after rain, that water needs somewhere to go — a low-profile sump in a dug pit, a drain to daylight, or corrected grading outside — and it needs it before you seal anything.

The order is: stop the bulk water, cover the ground, then manage what humidity is left.

A crawl space with a plastic vapor barrier over bare ground

In this section

How to do a crawl space in the right order

  1. Step 1Find where the water is coming fromWalk the outside perimeter during or just after rain. Downspouts discharging at the foundation and soil sloping back toward the house cause more crawl-space water than groundwater does, and both are cheap to fix.
  2. Step 2Give standing water a route outIf water pools after rain, dig a pit at the low point and set a pump with a low switch-on level, or run a drain to daylight if the ground falls away. This is the step people skip, and it is the one that makes everything after it work.
  3. Step 3Clear and prepare the groundRemove debris, old insulation and anything that will puncture the barrier. Rake the surface flat. Treat any active mold on joists before you cover the ground, because access gets much worse afterwards.
  4. Step 4Lay a 10-mil or heavier reinforced barrierOverlap seams by at least 12 inches and tape every lap. Run the barrier up the foundation walls by 6 to 12 inches and seal it to the wall with the manufacturer's tape or a mechanical fastener strip. Six-mil sheeting tears the first time you crawl on it.
  5. Step 5Decide vented or sealed, and commitA sealed and conditioned crawl space is the modern approach and generally performs better, but it requires sealing the vents, air-sealing the rim joist and providing either conditioned air or a dehumidifier. Half-sealing a crawl space — vents closed, no moisture control — is the one combination that reliably makes things worse.
  6. Step 6Add a dehumidifier sized for the spaceIn a sealed crawl space, a unit with a built-in condensate pump matters, because there is no gravity drain down there and nobody is emptying a bucket on their stomach. Target 50-55% relative humidity.
Plastic sheeting laid and taped across crawl space ground
An untaped lap is where the barrier fails first.

What the kits leave out

Encapsulation kits bundle barrier, tape, fasteners and sealant, and they are good value for the materials. What they consistently do not include is a water plan, a dehumidifier, or anything for the rim joist — which is where a large share of a crawl space's air leakage and condensation actually happens. Budget for all three separately and the kit becomes a fair purchase instead of a disappointing one.

Questions people ask

Does a crawl space need a sump pump?

Only if water collects there. If the ground is merely damp, a sealed vapor barrier and a dehumidifier handle it. If you see standing water or puddles after rain, you need a pit and a pump with a low switch-on level before you seal anything.

What thickness vapor barrier should I use?

10 mil reinforced is the usual floor for a crawl space anyone will ever enter; 12 to 20 mil is better where you will be moving around or storing anything. Six-mil sheeting is fine under a slab and poor on a crawl space floor.

Should crawl space vents be open or closed?

Modern practice in most US climates is to seal the vents and condition the space, because outdoor summer air brought into a cool crawl space condenses on everything. The important thing is to do it fully: sealing the vents without adding moisture control is worse than leaving them open.

Can I just put plastic down and skip the rest?

A loose-laid barrier is still much better than bare soil, so yes, it is worth doing on its own. But untaped laps curl up and the edges let vapor around the barrier, so taping and sealing roughly doubles what the same material achieves.

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